This study investigated the functional roles of the N-terminal Ca(2+) ion-binding sites, in terms of enzyme catalysis and stability, of peptidylarginine deiminase 4 (PAD4). Amino acid residues located in the N-terminal Ca(2+)-binding site of PAD4 were mutated to disrupt the binding of Ca(2+) ions. Kinetic data suggest that Asp155, Asp157 and Asp179, which directly coordinate Ca3 and Ca4, are essential for catalysis in PAD4. For D155A, D157A and D179A, the k(cat)/K(m,BAEE) values were 0.02, 0.63 and 0.01 s(-1)mM(-1) (20.8 s(-1)mM(-1) for WT), respectively. Asn153 and Asp176 are directly coordinated with Ca3 and indirectly coordinated with Ca5 via a water molecule. However, N153A displayed low enzymatic activity with a k(cat) value of 0.3 s(-1) (13.3 s(-1) for wild-type), whereas D176A retained some catalytic power with a k(cat) of 9.7 s(-1). Asp168 is the direct ligand for Ca5, and Ca5 coordination by Glu252 is mediated by two water molecules. However, mutation of these two residues to Ala did not cause a reduction in the k(cat)/K(m,BAEE) values, which indicates that the binding of Ca5 may not be required for PAD4 enzymatic activity. The possible conformational changes of these PAD4 mutants were examined. Thermal stability analysis of the PAD4 mutants in the absence or presence of Ca(2+) indicated that the conformational stability of the enzyme is highly dependent on Ca(2+) ions. In addition, the results of urea-induced denaturation for the N153, D155, D157 and D179 series mutants further suggest that the binding of Ca(2+) ions in the N-terminal Ca(2+)-binding site stabilizes the overall conformational stability of PAD4. Therefore, our data strongly suggest that the N-terminal Ca(2+) ions play critical roles in the full activation of the PAD4 enzyme.